During anaerobic digestion of organic substrates of agricultural, agri-food, municipal and industrial origin, renewable energy is produced in the form of biogas. Biogas contains methane, carbon dioxide and hydrogen sulphide in proportions ranging from 70% to 80%, 20% to 30%, and 0.1% to 4%, respectively. When biogas is used to produce heat or electrical energy, the presence of hydrogen sulphide in the biogas poses a major challenge. During combustion, this hydrogen sulphide is converted to sulphur oxide or sulphuric acid, which accelerates corrosion of equipment fueled by biogas, reduces its lifespan and substantially increases energy production costs. Current technologies (physical or chemical) for removing sulphur are expensive and not economically feasible at the farm scale.
On-farm methanation of agricultural waste has many environmental benefits, but also entails certain challenges in terms of adapting the technology to farm circumstances and scale. One of these challenges is the significant corrosive potential of biogas, primarily attributable to its water and hydrogen sulphide (H2S) content. The sulphur oxides (SO2) formed during the combustion of biogas can cause premature deterioration of biogas-fueled equipment and can also corrode structures in the vicinity of the bioreactors. In addition, after its release into the atmosphere, SO2 contributes to acid rain and, consequently, to forest degradation and loss of biodiversity.
H2S is produced during anaerobic digestion (AD) of municipal, agro-industrial or agricultural waste (Shchieder et al., 2003, Water Science Technology, 48(4): 209-212). The sulphur is present in the methionine and cysteine, two essential amino acids of animal and plant metabolism. Liquid animal manure is therefore rich in sulphur and produces a biogas with H2S concentrations as high as 6000 ppm. There are many biogas purification technologies available at both the experimental and commercial stages (Abatzoglou et al., 2005, Biofuels, Bioproducts & Biorefining, 3(1): 42-71; Jensen et al., 1999. Enzyme and microbial technology, 17: 2-10), but few are adaptable to the farm scale from a technical and economical perspective. The biological route therefore has many advantages and is the main focus of research in the field of biogas purification (Burgess et al., 2001. Biotechnology advances, 19: 35-63; Syed et al., 2006, Canadian Biosystems engineering, 48: 2.1-2.14).
One of the biological processes for controlling H2S emissions is to inject a limited quantity of air into the gas phase of the AD bioreactor. Under limited oxygen (O2) (microaerobic) conditions, microorganisms will promote the chemical reaction of oxidation of H2S into elemental sulphur (S0). In the presence of excess oxygen, microorganisms will instead promote the production of sulphates, a reaction with a higher energy yield. See the following equations:2HS−+O2→2S0+2OH−ΔG0=−169.35 kJ/mol  1)2HS−+4O2→2SO42−+2H+ΔG0=−732.58 kJ/mol  2)
O2 can be added either via the gas phase or via the liquid phase. The latter option requires a greater quantity of air, since part of the O2 will be consumed for aerobic oxidation of the organic matter (Jenicek et al., 2008, Water Science& Technology, 58(7): 1491-1496). The goal is to promote the action of facultative aerobic thiobacteria, normally present in AD sludges, without however adversely affecting the anaerobic process, the purpose of which is to produce methane. In fact, too much oxygen can inhibit the strictly anaerobic bacteria (Cirne et al., 2008, Rev Environmental Science & Biotechnology, 7: 93-105).
The crystalline form of this biologically produced sulphur is different from the form normally observed with chemical methods. These white or pale yellow orthorhombic crystals (S8) can be separated from the liquid fraction by sedimentation because of its higher density compared to water. Negatively charged polymer molecules are believed to bind to the S8 nuclei, which give the sulphur its hydrophobic properties (Janssen et al., 1999. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 151: 389-397). Solid sulphur is thus found in the bioreactor effluent and is available for use in agriculture along with the other nutrients (N, P, and K). This fertilizer can significantly enhance agricultural yields, particularly for vegetable crops (Grant et al., 2007, Canadian Journal of Plant Science, 87: 293-296).
In an AD bioreactor, sulphate-reducing bacteria (SRB) and hydrogenotrophic methanogens (HM) compete for the hydrogen available in the liquid phase. Dissolved hydrogen results from the hydrolysis and acidogenesis associated with the microbial activities. The SRB use this H2 to form H2S, which, in high concentrations, has an inhibitory effect on the HM as well as on the SRB. The specific methane yield can therefore be affected by high sulphur contents in the substrate being treated.
Jenicek et al. (2008, Water Science & technology, 58: 1491-1496) studied the impact of air dosing on two mesophilic AD bioreactors (BR1 and BR2) treating activated sludges. Air was injected into the sludge recirculation loop. The applied O2/S2− ratio was between 3.7 for BR1 and 5.5 for BR2. The H2S concentration without air injection was 3084 ppm for BR1 and 5338 ppm for BR2. The results indicated an average H2S reduction of 99% over several years of operation, with concentrations in the effluent of between 29 ppm and 50 ppm. It was also observed that microaerobic conditions did not reduce process performance. The specific CH4 yield for BR2 increased 50% following air injection and remained unchanged in the other bioreactor. For BR2, the ratio of volatile solids to total solids in the effluent was 65.8 in a strictly anaerobic environment and 59.7 in a microaerobic environment, and remained unchanged in BR1. To explain the improvement in the performance of BR2, it is hypothesized that the bioreactor was inhibited by high HS− concentrations before oxygenation started. This method relies on the control of the oxygen injection in function of the amount of sulphur present in the influent (ratio O/S−2). Sulphur analysis is an expensive and fairly complex analysis, which is not suited for agricultural application.
Khanal et al. (2003, Journal of environmental engineering, ASCE, 129: 1104-1111) used oxidation-reduction potential (ORP) as a controlling parameter to regulate O2 dosing in an upflow mesophilic AD biofilter system. Such filters are commonly employed in the treatment of waste water. Variable sulphate loads were applied (1000, 3000 and 6000 mg L−1) for a constant organic loading rate (18 g of chemical oxygen demand per liter). The reactor was initially operated under anaerobic conditions at a natural ORP (between −290 and −300 mV) and the ORP level was then increased by +25 mV through oxygenation of the liquid phase. It was demonstrated that O2 dosing reduced the sulphate concentration in the effluent by more than 98%. It was noted that the sulphur was primarily converted to the S0 form. It was also observed that part of the O2 was used for facultative aerobic processes and that this helped protect the methanogens from inhibition by the O2, particularly for lower sulphate loads. Hence, again under microaerobic conditions, methane production rates of 15.5% and 6.2% lower than the natural ORP level were observed for sulphate loads of 1,000 and 3,000 mg L−1, respectively. This study has been carried out with municipal wastes, which are not representative of agricultural wastes. For example, animal manures contain higher concentration of nitrogen that normally affects mesophilic anaerobic digestion processes when it exceeds 3000 to 4000 mg/L.
Van der Zee et al. (2007, Bioressource Technology, 98: 518-524) applied microaerobic conditions to an anaerobic fluidized bed bioreactor fed with vinasse at a sulphur loading rate of 1.3 mmol S d−1. Introduction of an air flow corresponding to an O2/S molar ratio of 8-10 (1.5 L d−1) was sufficient to reduce the H2S concentration in the effluent to undetectable levels (<0.02%). The oxidation reaction of the H2S appears to compete with aerobic organic matter breakdown processes. This article also described experiments under microaerobic conditions conducted in batch mode, the results of which demonstrated that the sulphur was oxidized primarily to the elemental form. The approach proposed in this study requires laboratory analysis of the substrate to quantify the sulfur concentration in the substrate. Also the substrate used is not representative of livestock manure.
There is still a need to be provided with method of removing hydrogen sulphide from biogas resulting from agriculture waste.
It would be thus highly desirable to be provided with a process that eliminates hydrogen sulphide from biogas resulting from agriculture waste that is low in cost, is very stable, simple, easy to operate and which does not interfere with regular farm operations.